This application claims the benefit of Korean Patent Application No. 1999-67847, filed on Dec. 31, 2000, under 35 U.S.C. xc2xa7119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having a retardation film.
2. Description of Related Art
LCD devices are divided into three types: a transmissive LCD device; a reflective LCD device; and a transflective LCD device. The transmissive LCD device has a back light device as a light source, and the reflective LCD device uses ambient light instead of the back light device. The transflective LCD device has a transmissive mode and a reflective mode.
FIG. 1 is an exploded view illustrating a typical transflective LCD device. As shown in FIG. 1, the transflective LCD device 1 includes upper and lower substrates 7 and 22 with a liquid crystal layer 15 interposed therebetween. The upper substrate 7 includes a black matrix 3, a color filter 5, and a common electrode 9. The lower substrate 22 is called an array substrate and includes gate lines 21 arranged in a transverse direction and data lines 26 arranged in a longitudinal direction perpendicular to the gate line 21. A pixel region P is defined by the gate and data lines 21 and 26. A pixel electrode 18 is formed on the pixel region P. As a switching element, thin film transistors (TFTs) T are formed at a crossing point of the gate and data lines 21 and 26. The pixel electrode 18 is made of indium tin oxide (ITO) or indium zinc oxide (IZO).
FIG. 2 is a cross-sectional view illustrating a conventional reflective LCD device. As shown in FIG. 2, a lower substrate 11 includes a reflective electrode 16, and an upper substrate 13 includes a transparent electrode 17. A liquid crystal layer 19 is interposed between the lower and upper substrates 11 and 13. As the liquid crystal layer 19, twisted nematic (TN) liquid crystal is mainly used. The twisted nematic liquid crystal molecules have a molecule axis that is twisted in 90xc2x0 and a phase difference of xcex/4. Therefore, a linearly polarized light incident to the twisted nematic liquid crystal layer is changed into a circularly polarized light. On the upper substrate 13, a retardation film 23 (i.e., a quarter wave plate) and a polarizer 25 are arranged. Ambient light is changed into a linearly polarized light through the polarizer 25. The retardation film 23 changes the linearly polarized light into a circularly polarized light.
FIG. 3A shows light state after passing through each layer when voltage is not applied to the liquid crystal layer and assuming that the observer traces light. Incident light passes through the polarizer 25 and become a linearly polarized light parallel to the light transmission axis of the polarizer 25. The linearly polarized light is changed into a left-handed circularly polarized light after passing through the retardation film 23. The left-handed circularly polarized light is changed into the linearly polarized light after passing the TN liquid crystal layer 19. The linearly polarized light is reflected on the reflective electrode 16 and is changed into the left-handed circularly polarized light having a phase difference of xcex/4after passing through the TN liquid crystal layer 19. The left-handed circularly polarized light is changed into the linearly polarized light after passing through the retardation film 23. The linearly polarized light is parallel to a transmission axis of the polarizer 25 and thus passes through the polarizer 25, whereupon the LCD device becomes a white state.
Alternately, FIG. 3B shows light state after passing through each layer when voltage is applied to the liquid crystal layer. When the liquid crystal display device goes to an on state, the TN liquid crystal molecules that are twisted in 90xc2x0 are polarized in a direction of applied electric field to have a certain direction. As shown in FIG. 3B, incident light passes through the polarizer 25 and become a linearly polarized light parallel to the light transmission axis of the polarizer 25. The linearly polarized light is changed into a left-handed circularly polarized light after passing through the retardation film 23. The left-handed circularly polarized light passes through the TN liquid crystal layer 19 xe2x80x9cas isxe2x80x9d. The left-handed circularly polarized light is reflected on the reflective electrode 16 and is changed into the right-handed circularly polarized light. The right-handed circularly polarized light passes through the TN liquid crystal layer 19 xe2x80x9cas isxe2x80x9d. The right-handed circularly polarized light is changed into the linearly polarized light after passing through the retardation film 23. The linearly polarized light is parallel to a transmission axis of the polarizer 25 and thus passes through the polarizer 25, whereupon the LCD device becomes a dark state.
The retardation film 23 serves to improve a viewing angle and a contrast ratio. Even though the thickness of the retardation film 23 is changed a little, error of the phase difference become greater. Further, when it is assembled to the substrate, assembly or attachment error may occur, leading to an assembling error of the polarizer.
FIG. 4 is a cross-sectional view illustrating a transflective LCD device according to the conventional art. As shown in FIG. 4, on an upper substrate 43, an upper retardation film 42 and an upper polarizer 45 are sequentially stacked. On a bottom surface of a lower substrate 53, a lower polarizer 47 and a lower retardation film 50 are arranged. A liquid crystal layer 55 are interposed between the upper and lower substrates 43 and 47. A back light device is arranged under the lower substrate 53. A reflective electrode 49 is arranged on the lower substrate 53 and includes at least one light transmitting hole 51. The light transmitting hole 51 serves to transmit light from the back light device 41.
The transflective LCD device described above requires the lower polarizer 47 as well as the upper polarizer 45. This is because light should not leak out in both a transmissive mode and a reflective mode when the transflective LCD device goes to a dark state in order to achieve a high contrast ratio.
FIG. 5A shows light state after passing through each layer in a transmissive mode when voltage is applied to the liquid crystal layer 55, and assuming that the lower retardation film 50 does not exist. As shown in FIG. 5A, light generated from the back light device 41 passes through the lower polarizer 47 and is changed into a linearly polarized light parallel to a transmission axis of the lower polarizer 47. The linearly polarized light passes through the light transmitting hole 51 and the liquid crystal layer 55 xe2x80x9cas isxe2x80x9d. In other words, the linearly polarized light passing through the liquid crystal layer 55 has no phase difference. Then, the linearly polarized light passes through the upper retardation film 42 and is changed into left-handed circularly polarized light. The left-handed circularly polarized light passes through the upper polarizer 45. At this point, of the left-handed circularly polarized light, only elements of light parallel to a transmission axis of the upper polarizer 45 are viewed by an observer. In other words, about a half of the circularly polarized light passes through the upper polarizer 45, leading to a gray state other than a complete dark state, leading to a deterioration of contrast ratio.
FIG. 5B shows light state after passing through each layer in a transmissive mode when voltage is applied to the liquid crystal layer 55 and the lower retardation film 50 is arranged. As shown in FIG. 5B, light generated from the back light device 41 passes through the lower polarizer 47 and is changed into a linearly polarized light parallel to a transmission axis of the lower polarizer 47. The linearly polarized light passes through the lower retardation film 50 and is changed into a right-handed circularly polarized light. The right-handed circularly polarized light passes through the light transmitting hole 51 and the liquid crystal layer 55 xe2x80x9cas isxe2x80x9d. Then, the right-handed circularly polarized light passes through the upper retardation film 42 and is changed into a linearly polarized light perpendicular to a transmission axis of the upper polarizer 45. The linearly polarized light is absorbed into the upper polarizer 45, leading to a dark state.
In the transmissive mode of the transflective LCD device, light D generated from the back light device 41 passes through the light transmitting hole 51 and reaches the liquid crystal layer 55. However, part C of light generated from the back light device 41 is reflected on the reflective electrode 49 and then is absorbed into the lower polarizer 47. Or a small amount of light is reflected on the reflective electrode 49 and then directs toward the liquid crystal layer 55 through the light transmitting hole 51.
FIG. 6 shows a state of light C reflected on the reflective electrode in a transmissive mode. As shown in FIG. 6, light C from the back light device 41 passes through the lower polarizer 47 and is changed into a linearly polarized light parallel to a transmission axis of the lower polarizer 47. The linearly polarized light is changed into a left-handed circularly polarized light through the retardation film 50 that is a xcex/4 plate. The left-handed circularly polarized light is reflected on the reflective electrode 49 and is changed into a right-handed circularly polarized light. The right-handed circularly polarized light passes through the lower retardation film 50 and is changed into the linearly polarized light perpendicular to a transmission axis of the lower polarizer 47. The linearly polarized light is all absorbed into the lower polarizer 47. Therefore, the lower retardation film 50 causes light lose in the transmissive mode of the transflective LCD device, whereupon a brightness may be lowered.
To overcome the problems described above, preferred embodiments of the present invention provide a reflective liquid crystal display (LCD) device having the assembly improvement of a retardation film.
It is another object of the present invention to provide a transflective LCD device having a high brightness and an excellent color purity.
In order to achieve the above object, the preferred embodiments of the present invention provide a reflective liquid crystal display device, including: a first substrate having a reflective electrode on a bottom surface thereof, a second substrate having a polarizer and a retardation film, the polarizer formed on a top surface of the second substrate, the retardation film formed on a bottom surface of the second substrate; and a liquid crystal layer interposed between the first and second substrates, wherein the retardation film is made of one of a polymer and a liquid crystal.
The preferred embodiment of the present invention further provides a transflective liquid crystal display, including: a liquid crystal panel including a) a first substrate having a first polarizer and a reflective electrode, the first polarizer formed a bottom surface thereof, the reflective electrode having at least one light transmitting hole and a first retardation film, the first retardation film formed in the light transmitting hole, the light transmitting hole transmitting light; b) a second substrate having a second retardation film and a second polarizer sequentially arranged thereon; and c) a liquid crystal layer interposed between the first and second substrates; and a back light device for generating light.
The preferred embodiment of the present invention further provides a liquid crystal display device, including: a liquid crystal panel including: a) an upper substrate having an upper polarizer and an upper retardation film sequentially arranged on a top surface thereof; b) a lower substrate having a reflective electrode and a first lower retardation film sequentially arranged on a top surface thereof and a second lower retardation film and a lower polarizer sequentially arranged on a bottom surface thereof, the reflective electrode and the first lower retardation film having a light transmitting hole, the light transmitting hole transmitting light; and c) a liquid crystal layer interposed between the upper and lower substrates; and a back light device providing light to the liquid crystal panel.
The retardation film is made of either of an UV curable polymer and an UV curable liquid crystal.